Post on 04-Jan-2016
1
Chapter 7. Spread Spectrum
Wen-Shyang HwangKUAS EE.
2
Spread Spectrum
Input is fed into a channel encoder Produces analog signal with narrow bandwidth
Signal is further modulated using sequence of digits Spreading code or spreading sequence Generated by pseudonoise, or pseudo-random number generator
Effect of modulation is to increase bandwidth of signal to be transmitted
On receiving end, digit sequence is used to demodulate the spread spectrum signal
Signal is fed into a channel decoder to recover data
3
Spread Spectrum
What can be gained from apparent waste of spectrum? Immunity from various kinds of noise and multipath distortion Can be used for hiding and encrypting signals Several users can independently use the same higher bandwidth
with very little interference
4
Frequency Hoping Spread Spectrum (FHSS)
Signal is broadcast over seemingly random series of radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of input signal
Signal hops from frequency to frequency at fixed intervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency is selected
Channel sequence dictated by spreading code Receiver, hopping between frequencies in synchronization with
transmitter, picks up message Advantages
Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only at knocking
out a few bits
5
Frequency Hoping Spread Spectrum
6
FHSS Using MFSK
MFSK signal is translated to a new frequency every Tc seconds by modulating the MFSK signal with the FHSS carrier signal
For data rate of R: duration of a bit: T = 1/R seconds duration of signal element: Ts = LT seconds
Tc Ts - slow-frequency-hop spread spectrum Tc < Ts - fast-frequency-hop spread spectrum
7
FHSS Using MFSK
8
FHSS Performance Considerations
Large number of frequencies used Results in a system that is quite resistant to jamming
Jammer must jam all frequencies With fixed power, this reduces the jamming power in any one frequency
band
9
Direct Sequence Spread Spectrum (DSSS)
Each bit in original signal is represented by multiple bits in the transmitted signal
Spreading code spreads signal across a wider frequency band Spread is in direct proportion to number of bits used
One technique combines digital information stream with the spreading code bit stream using exclusive-OR (Figure 7.6)
10
Direct Sequence Spread Spectrum (DSSS)
11
DSSS Using BPSK
Multiply BPSK signal,sd(t) = A d(t) cos(2 fct)
by c(t) [takes values +1, -1] to gets(t) = A d(t)c(t) cos(2 fct)
A = amplitude of signal fc = carrier frequency d(t) = discrete function [+1, -1]
At receiver, incoming signal multiplied by c(t) Since, c(t) x c(t) = 1, incoming signal is recovered
s(t) c(t) = A d(t) c(t) c(t) cos(2 fct) = sd(t)
12
DSSS Using BPSK
13
Code-Division Multiple Access (CDMA)
Basic Principles of CDMA D = rate of data signal Break each bit into k chips
Chips are a user-specific fixed pattern Chip data rate of new channel = kD
14
CDMA Example
If k=6 and code is a sequence of 1s and -1s For a ‘1’ bit, A sends code as chip pattern
<c1, c2, c3, c4, c5, c6> For a ‘0’ bit, A sends complement of code
<-c1, -c2, -c3, -c4, -c5, -c6> Receiver knows sender’s code and performs electronic decode
function
<d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender’s code
665544332211 cdcdcdcdcdcddSu
15
CDMA Example
User A code = <1, –1, –1, 1, –1, 1> To send a 1 bit = <1, –1, –1, 1, –1, 1> To send a 0 bit = <–1, 1, 1, –1, 1, –1>
User B code = <1, 1, –1, – 1, 1, 1> To send a 1 bit = <1, 1, –1, –1, 1, 1>
Receiver receiving with A’s code (A’s code) x (received chip pattern)
User A ‘1’ bit: 6 -> 1 User A ‘0’ bit: -6 -> 0 User B ‘1’ bit: 0 -> unwanted signal ignored
If A and B transmit signals SA and SB at the same time SA(SA+SB) = SA(SA) + SA(SB) = SA(SA)
Orthogonal: The codes of A and B that have the property that SA(cB) = SB(cA) = 0
16
CDMA Example
17
CDMA for Direct Sequence Spread Spectrum
18
Categories of Spreading Sequences
Spreading Sequence Categories PN sequences Orthogonal codes
For FHSS systems PN sequences most common
For DSSS systems not employing CDMA PN sequences most common
For DSSS CDMA systems PN sequences Orthogonal codes
19
PN Sequences
PN generator produces periodic sequence that appears to be random
PN Sequences Generated by an algorithm using initial seed Sequence isn’t statistically random but will pass many test of
randomness Sequences referred to as pseudorandom numbers or pseudonoise
sequences Unless algorithm and seed are known, the sequence is impractical
to predict
20
Important PN Properties
Randomness Uniform distribution
Balance property Run property
Independence Correlation property
Unpredictability
21
Linear Feedback Shift Register Implementation
22
23
Maximal-Length Shift Register Sequences
24
Definitions
Correlation The concept of determining how much similarity one set of data
has with another Range between –1 and 1
1 The second sequence matches the first sequence 0 There is no relation at all between the two sequences -1 The two sequences are mirror images
Cross correlation The comparison between two sequences from different sources
rather than a shifted copy of a sequence with itself
25
Advantages of Cross Correlation
The cross correlation between an m-sequence and noise is low This property is useful to the receiver in filtering out noise
The cross correlation between two different m-sequences is low This property is useful for CDMA applications Enables a receiver to discriminate among spread spectrum signals
generated by different m-sequences
26
Gold Sequences
Gold sequences constructed by the XOR of two m-sequences with the same clocking
Codes have well-defined cross correlation properties Only simple circuitry needed to generate large number of unique cod
es In following example (Figure 7.16a) two shift registers generate the t
wo m-sequences and these are then bitwise XORed
27
Orthogonal Codes
Orthogonal codes All pairwise cross correlations are zero Fixed- and variable-length codes used in CDMA systems For CDMA application, each mobile user uses one sequence in the set as
a spreading code Provides zero cross correlation among all users
Types Welsh codes Variable-Length Orthogonal codes